Channel quality feedback mechanism and method

ABSTRACT

Methods and apparatus are presented for improving the feedback of channel information to a serving base station, which allows a reduction in the reverse link load while allowing the base station to improve the forward link data throughput. Over a channel quality indicator channel, three subchannels are generated; the re-synch subchannel, the differential feedback subchannel, and the transition indicator subchannel. The information carried on each subchannel can be used separately or together by a base station to selectively update internal registers storing channel conditions. The channel conditions are used to determine transmission formats, power levels, and data rates of forward link transmissions.

BACKGROUND

1. Field

The present invention relates generally to communications, and morespecifically, to improving the feedback of channel information, whichcan be used to improve the scheduling and rate control of traffic over awireless communication system.

2. Background

The field of wireless communications has many applications including,e.g., cordless telephones, paging, wireless local loops, personaldigital assistants (PDAs), Internet telephony, and satellitecommunication systems. A particularly important application is cellulartelephone systems for mobile subscribers. As used herein, the term“cellular” system encompasses both cellular and personal communicationservices (PCS) frequencies. Various over-the-air interfaces have beendeveloped for such cellular telephone systems including, e.g., frequencydivision multiple access (FDMA), time division multiple access (TDMA),and code division multiple access (CDMA). In connection therewith,various domestic and international standards have been establishedincluding, e.g., Advanced Mobile Phone Service (AMPS), Global System forMobile (GSM), and Interim Standard 95 (IS-95). IS-95 and itsderivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred tocollectively herein as IS-95), and proposed high-data-rate systems arepromulgated by the Telecommunication Industry Association (TIA) andother well known standards bodies.

Cellular telephone systems configured in accordance with the use of theIS-95 standard employ CDMA signal processing techniques to providehighly efficient and robust cellular telephone service. Exemplarycellular telephone systems configured substantially in accordance withthe use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459and 4,901,307, which are assigned to the assignee of the presentinvention and incorporated by reference herein. An exemplary systemutilizing CDMA techniques is the cdma2000 ITU-R Radio TransmissionTechnology (RTT) Candidate Submission (referred to herein as cdma2000),issued by the TIA. The standard for cdma2000 is given in the draftversions of IS-2000 and has been approved by the TIA and 3GPP2. AnotherCDMA standard is the W-CDMA standard, as embodied in 3^(rd) GenerationPartnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3GTS 25.213, and 3G TS 25.214.

The telecommunication standards cited above are examples of only some ofthe various communication systems that can be implemented. Some of thesevarious communication systems are configured so that remote stations cantransmit information regarding the quality of the transmission medium toa serving base station. This channel information can then be used by theserving base station to optimize the power levels, the transmissionformats, and the timing of forward link transmissions, and further, tocontrol the power levels of reverse link transmissions.

As used herein, “forward link” refers to the transmissions directed froma base station to a remote station and “reverse link” refers totransmissions directed from a remote station to a base station. Theforward link and the reverse link are uncorrelated, meaning thatobservations of one do not facilitate the prediction of the other.However, for stationary and slow-moving remote stations, thecharacteristics of the forward link transmission path will be observedto be similar to the characteristics of the reverse link transmissionpath in a statistical sense.

Channel conditions of received forward link transmissions, such as thecarrier-to-interference (C/I) ratio, can be observed by a remotestation, which reports such information to a serving base station. Thebase station then uses this knowledge to schedule transmissions to theremote station selectively. For example, if the remote station reportsthe presence of a deep fade, the base station would refrain fromscheduling a transmission until the fading condition passes.Alternatively, the base station may decide to schedule a transmission,but at a high transmission power level in order to compensate for thefading condition. Alternatively, the base station may decide to alterthe data rate at which transmissions are sent, by transmitting data informats that can carry more information bits. For example, if thechannel conditions are bad, data can be transmitted in a transmissionformat with redundancies so that corrupted symbols are more likely to berecoverable. Hence, the data throughput is lower than if a transmissionformat without redundancies were used instead.

The base station can also use this channel information to balance thepower levels of all the remote stations within operating range, so thatreverse link transmissions arrive at the same power level. In CDMA-basedsystems, channelization between remote stations is produced by the useof pseudorandom codes, which allows a system to overlay multiple signalson the same frequency. Hence, reverse link power control is an essentialoperation of CDMA-based systems because excess transmission poweremitted from one remote station could “drown out” transmissions of itsneighbors.

In communication systems that use feedback mechanisms to determine thequality of the transmission media, channel conditions are continuouslyconveyed on the reverse link. This produces a large load upon thesystem, consuming system resources that could otherwise be allocated toother functions. Hence, there is a need to reduce the reverse link loadof unnecessary transmissions, which can occur when the remote stationstransmit C/I information that have not changed substantially from theprevious transmissions. However, the system must still be able to detectand react to changing channel conditions in a timely manner. Theembodiments described herein address these needs by providing amechanism for optimizing the transmission of channel information on thereverse link and for decoding such information at a base station.

SUMMARY

Methods and apparatus are presented herein to address the needs statedabove. In one aspect, an apparatus is presented for scheduling forwardlink transmissions, the apparatus comprising: a memory element; and aprocessing element configured to execute a set of instructions stored onthe memory element, the set of instructions for: receiving a fullchannel quality value and a plurality of incremental channel qualityvalues from a remote station, wherein the plurality of incrementalchannel quality values are received sequentially; and selectivelyupdating a register with a channel quality estimate, wherein the channelquality estimate is based upon the full channel quality value and theplurality of incremental channel quality values.

In another aspect, a method is presented for estimating forward linkchannel quality from a full channel quality value and a plurality ofincremental channel quality values, the method comprising: decoding thefull channel quality value over a plurality of slots; incrementallyupdating a channel state register with the plurality of incrementalchannel quality values, wherein each of the plurality of incrementalchannel quality values are received separately over each of theplurality of slots; and resetting the channel state register with thefull channel quality value when the full channel quality value is fullydecoded.

In another aspect, an apparatus is presented for transmitting channelquality values over a feedback channel to a base station, the apparatuscomprising: a re-synch subchannel generation system for generating fullchannel quality values; and a differential feedback subchannelgeneration system for generating a plurality of incremental values,wherein the plurality of incremental values are multiplexed with thefull channel quality values.

In another aspect, a method is presented for transmitting channelinformation from a remote station to a base station, the methodcomprising: generating a full channel quality value; and generating anincremental channel quality value, wherein the incremental channelquality value is multiplexed with the full channel quality value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a wireless communication network.

FIG. 2A, FIG. 2B, and FIG. 2C are timelines that describe theinteractions between the re-synch subchannel and the differentialfeedback subchannel.

FIG. 3 is a functional block diagram of a remote station incommunication with a base station.

FIG. 4A, FIG. 4B, FIG. 4C, and FIG. 4D are timelines of differentimplementations of the re-synch subchannel and the differential feedbacksubchannel.

FIG. 4E is a table illustrating different values arising from differentinterpretations of the information received on the re-synch subchanneland the differential feedback subchannel.

FIG. 5 is a graph illustrating an advantage of the “accumulate and add”method when a deep fade occurs.

FIG. 6A and FIG. 6B are block diagrams of channel elements forgenerating re-synch subchannel, the differential feedback subchannel,and the transition indicator subchannel.

FIG. 7 is a graph illustrating an advantage of using the re-synchsubchannel and the differential feedback subchannel at quantizationlimits.

DETAILED DESCRIPTION

As illustrated in FIG. 1, a wireless communication network 10 maygenerally includes a plurality of mobile stations (also called remotestations or subscriber units or user equipment) 12 a–12 d, a pluralityof base stations (also called base station transceivers (BTSs) or NodeB). 14 a–14 c, a base station controller (BSC) (also called radionetwork controller or packet control function 16), a mobile switchingcenter (MSC) or switch 18, a packet data serving node (PDSN) orinternetworking function (IWF) 20, a public switched telephone network(PSTN) 22 (typically a telephone company), and an Internet Protocol (IP)network 24 (typically the Internet). For purposes of simplicity, fourmobile stations 12 a–12 d, three base stations 14 a–14 c, one BSC 16,one MSC 18, and one PDSN 20 are shown. It would be understood by thoseskilled in the art that there could be more or less number of mobilestations 12, base stations 14, BSCs 16, MSCs 18, and PDSNs 20.

In one embodiment the wireless communication network 10 is a packet dataservices network. The mobile stations 12 a–12 d may be any of a numberof different types of wireless communication device such as a portablephone, a cellular telephone that is connected to a laptop computerrunning IP-based, Web-browser applications, a cellular telephone withassociated hands-free car kits, a personal data assistant (PDA) runningIP-based, Web-browser applications, a wireless communication moduleincorporated into a portable computer, or a fixed location communicationmodule such as might be found in a wireless local loop or meter readingsystem. In the most general embodiment, mobile stations may be any typeof communication unit.

The mobile stations 12 a–12 d may advantageously be configured toperform one or more wireless packet data protocols such as described in,for example, the EIA/TIA/IS-707 standard. In a particular embodiment,the mobile stations 12 a–12 d generate IP packets destined for the IPnetwork 24 and encapsulate the IP packets into frames using apoint-to-point protocol (PPP).

In one embodiment the IP network 24 is coupled to the PDSN 20, the PDSN20 is coupled to the MSC 18, the MSC is coupled to the BSC 16 and thePSTN 22, and the BSC 16 is coupled to the base stations 14 a–14 c viawirelines configured for transmission of voice and/or data packets inaccordance with any of several known protocols including, e.g., E1, T1,Asynchronous Transfer Mode (ATM), IP, PPP, Frame Relay, HDSL, ADSL, orxDSL. In an alternate embodiment, the BSC 16 can be coupled directly tothe PDSN 20.

During typical operation of the wireless communication network 10, thebase stations 14 a–14 c receive and demodulate sets of reverse signalsfrom various mobile stations 12 a–12 d engaged in telephone calls, Webbrowsing, or other data communications. Each reverse signal received bya given base station 14 a–14 c is processed within that base station 14a–14 c. Each base station 14 a–14 c may communicate with a plurality ofmobile stations 12 a–12 d by modulating and transmitting sets of forwardsignals to the mobile stations 12 a–12 d. For example, as shown in FIG.1, the base station 14 a communicates with first and second mobilestations 12 a, 12 b simultaneously, and the base station 14 ccommunicates with third and fourth mobile stations 12 c, 12 dsimultaneously. The resulting packets are forwarded to the BSC 16, whichprovides call resource allocation and mobility management functionalityincluding the orchestration of soft handoffs of a call for a particularmobile station 12 a–12 d from one base station 14 a–14 c to another basestation 14 a–14 c. For example, a mobile station 12 c is communicatingwith two base stations 14 b, 14 c simultaneously. Eventually, when themobile station 12 c moves far enough away from one of the base stations14 c, the call will be handed off to the other base station 14 b.

If the transmission is a conventional telephone call, the BSC 16 willroute the received data to the MSC 18, which provides additional routingservices for interface with the PSTN 22. If the transmission is apacket-based transmission such as a data call destined for the IPnetwork 24, the MSC 18 will route the data packets to the PDSN 20, whichwill send the packets to the IP network 24. Alternatively, the BSC 16will route the packets directly to the PDSN 20, which sends the packetsto the IP network 24.

In some communication systems, packets carrying data traffic are dividedinto subpackets, which occupy slots of a transmission channel. Forillustrative ease only, the nomenclature of a cdma2000 system is usedhereafter. Such use is not intended to limit the implementation of theembodiments herein to cdma2000 systems. Implementations in othersystems, such as, e.g., WCDMA, can be accomplished without affecting thescope of the embodiments described herein.

The forward link from the base station to a remote station operatingwithin the range of the base station can comprise a plurality ofchannels. Some of the channels of the forward link can include, but arenot limited to a pilot channel, synchronization channel, paging channel,quick paging channel, broadcast channel, power control channel,assignment channel, control channel, dedicated control channel, mediumaccess control (MAC) channel, fundamental channel, supplemental channel,supplemental code channel, and packet data channel. The reverse linkfrom a remote station to a base station also comprises a plurality ofchannels. Each channel carries different types of information to thetarget destination. Typically, voice traffic is carried on fundamentalchannels, and data traffic is carried on supplemental channels or packetdata channels. Supplemental channels are usually dedicated channels,while packet data channels usually carry signals that are designated fordifferent parties in a time and/or code-multiplexed manner.Alternatively, packet data channels are also described as sharedsupplemental channels. For the purposes of describing the embodimentsherein, the supplemental channels and the packet data channels aregenerically referred to as data traffic channels.

Voice traffic and data traffic are typically encoded, modulated, andspread before transmission on either the forward or reverse links. Theencoding, modulation, and spreading can be implemented in a variety offormats. In a CDMA system, the transmission format ultimately dependsupon the type of channel over which the voice traffic and data trafficare being transmitted and the condition of the channel, which can bedescribed in terms of fading and interference.

Predetermined transmit formats, which correspond to a combination ofvarious transmit parameters, can be used to simplify the choice oftransmission formats. In one embodiment, the transmission formatcorresponds to a combination of any or all of the following transmissionparameters: the modulation scheme used by the system, the number oforthogonal or quasi-orthogonal codes, an identification of theorthogonal or quasi-orthogonal codes, the data payload size in bits, theduration of the message frame, and/or details regarding the encodingscheme. Some examples of modulation schemes used within communicationsystems are the Quadrature Phase Shift Keying scheme (OPSK), 8-ary PhaseShift Keying scheme (8-PSK), and 16-ary Quadrature Amplitude Modulation(16-QAM). Some of the various encoding schemes that can be selectivelyimplemented are convolutional encoding schemes, which are implemented atvarious rates, or turbo coding, which comprises multiple encoding stepsseparated by interleaving steps.

Orthogonal and quasi-orthogonal codes, such as the Walsh code sequences,are used to channelize the information sent to each remote station. Inother words, Walsh code sequences are used on the forward link to allowthe system to overlay multiple users, each assigned one or severaldifferent orthogonal or quasi-orthogonal codes, on the same frequencyduring the same time duration.

A scheduling element in the base station is configured to control thetransmission format of each packet, the rate of each packet, and theslot times over which each packet is to be transmitted to a remotestation. The terminology “packet” is used to describe system traffic.Packets can be divided into subpackets, which occupy slots of atransmission channel. “Slot” is used to describe a time duration of amessage frame. The use of such terminology is common in cdma2000systems, but the use of such terminology is not meant to limit theimplementation of the embodiments herein to cdma2000 systems.Implementation in other systems, such as, e.g. WCDMA, can beaccomplished without affecting the scope of the embodiments describedherein.

Scheduling is a vital component in attaining high data throughput in apacket-based system. In the cdma2000 system, the scheduling element(which is also referred to as a “scheduler” herein) controls the packingof payload into redundant and repetitious subpackets that can besoft-combined at a receiver, so that if a received subpacket iscorrupted, it can be combined with another corrupted subpacket todetermine the data payload within an acceptable frame error rate (FER).For example, if a remote station requests the transmission of data at76.8 kbps, but the base station knows that this transmission rate is notpossible at the requested time due to the condition of channel, thescheduler in the base station can control the packaging of the datapayload into multiple subpackets. The remote station will receivemultiple corrupted subpackets, but will still be likely to recover thedata payload by soft-combining the uncorrupted bits of the subpackets.Hence, the actual transmission rate of the bits can be different fromthe data throughput rate.

The scheduling element in the base station uses an open-loop algorithmto adjust the data rate and scheduling of forward link transmissions.The open-loop algorithm adjusts transmissions in accordance with thevarying channel conditions typically found in a wireless environment. Ingeneral, a remote station measures the quality of the forward linkchannel and transmits such information to the base station. The basestation uses the received channel conditions to predict the mostefficient transmission format, rate, power level and timing of the nextpacket transmission. In the cdma2000 1×EV-DV system, the remote stationscan use a channel quality feedback channel (CQICH) to convey channelquality measurements of the best serving sector to the base station. Thechannel quality may be measured in terms of a carrier-in-interference(C/I) ratio and is based upon received forward link signals. The C/Ivalue is mapped onto a five-bit channel quality indicator (CQI) symbol,wherein the fifth bit is reserved. Hence, the C/I value can have one ofsixteen quantization values.

Since the remote station is not prescient, the remote station transmitsthe C/I values continuously, so that the base station is aware of thechannel conditions if ever any packets need to be transmitted on theforward link to that remote station. The continuous transmission of4-bit C/I values consumes the battery life of the remote station byoccupying hardware and software resources in the remote station.

In addition to the problems of battery life and reverse link loading,there is also a problem of latency. Due to propagation and processingdelays, the base station is scheduling transmissions using outdatedinformation. If the typical propagation delay is 2.5 ms in duration,which corresponds to a 2-slot delay in systems with 1.25 ms slots, thenthe base station may be reacting to a situation that no longer exists,or may fail to react in a timely manner to a new situation.

For the above reasons, the communication network requires a mechanism toconvey information to the base station that allows the base station toquickly reschedule transmissions due to sudden changes in the channelenvironment. In addition, the aforementioned mechanism should reduce thedrain on battery life of the remote station and the load on the reverselink.

The embodiments described herein are directed to improving the feedbackmechanism for conveying channel information, such as C/I, from theremote station to the base station while reducing the load of thereverse link. By improving the feedback mechanism, the embodimentsimprove the ability of a base station to schedule transmissions and thedata rates of the transmissions in accordance with actual channelconditions. The embodiments are directed toward generating twosubchannels on the CQI channel in order to carry channel information. Itshould be noted that other channels could also be configured to carrythe subchannels described herein, but for the sake of expediency, theterminology of the CQI channel is used henceforth. The two subchannelsare referred to hereafter as the re-synch subchannel and thedifferential feedback subchannel.

In addition to improvements in the feedback mechanism at the remotestation, improvements at the base station can also be implemented tooptimize the interpretation of the channel information received from theremote station. A scheduling element in the base station can beconfigured to implement task functions in accordance with informationreceived from either subchannel or by selectively discarding informationreceived from either subchannel.

In a general description of the embodiments, full C/I values aretransmitted on the re-synch subchannel while incremental 1-bit valuesare transmitted over the differential feedback subchannel. Theincremental 1-bit values of 1 and 0 are mapped to +0.5 dB and −0.5 dB,but can be mapped to other values ±K as well, where K is a systemdefined step size.

Generation of Subchannels at a Remote Station

The values sent on the re-synch and differential feedback subchannelsare determined based on the forward link C/I measurements. The valuesent on the re-sync subchannel is obtained by quantizing the most recentC/I measurement. A one-bit value is sent on the differential feedbacksubchannel and is obtained by comparing the most recent C/I measurementwith the contents of an internal register. The internal register isupdated based on past values sent on the re-synch and differentialfeedback subchannels, and represents the remote station's best estimateof the C/I value that the base station will decode.

In a first mode, channel elements can be placed within a remote stationto generate the re-synch subchannel and the differential feedbacksubchannel over the CQI channel (CQICH), wherein the re-synch subchanneloccupies one slot of an N-slot CQICH frame and the differential feedbacksubchannel occupies all slots of the N-slot CQICH frame, so that anincremental 1-bit value is transmitted in each slot. Hence, in at leastone slot of the N-slot CQICH frame, both a full C/I value and anincremental 1-bit value are transmitted to the base station. Thisconcurrent transmission is possible through the use of orthogonal orquasi-orthogonal spreading codes, or in an alternative embodiment, bytime interleaving the two subchannels in some predetermined fashion. Inan alternate first mode, the re-synch subchannel and the differentialfeedback subchannel are not sent in parallel. Instead, the re-synchsubchannel is transmitted over one slot and the system refrains fromtransmitting the differential feedback subchannel in that particularslot. FIG. 2A is a timeline illustrating the transmission timing of there-synch channel and the differential feedback subchannel operating inparallel in the first mode.

In a second mode, the channel elements are configured so that the twosubchannels are generated with the re-synch subchannel operating at areduced rate. The re-synch channel operates at a reduced rate when afull C/I value is spread over at least two slots of an N-slot CQICHframe. For example, the full C/I value may be transmitted at a reducedrate over 2, 4, 8, or 16 slots of a 16-slot CQICH frame. Thedifferential feedback subchannel occupies all of the slots of the N-slotCQICH frame. Hence, an incremental 1-bit value is transmitted in eachslot, in parallel to the re-synch subchannel. The remote station shouldtransmit the full C/I value at the reduced rate when the reverse link issuffering from unfavorable channel conditions. In one embodiment, thebase station determines the reverse link channel conditions andtransmits a control signal to the remote station, wherein the controlsignal informs the remote station as to whether the re-synch subchannelshould operate at a reduced rate or not. Alternatively, the remotestation can be programmed to make this determination independently.

In one implementation of the second mode, the two subchannels workparallel at a reduced rate wherein a full C/I value is spread over allslots of a N-slot CQICH frame and each slot also carries an incremental1-bit value. In an alternate second mode, the differential feedbacksubchannel occupies all of the slots of the N-slot frame except for thefirst slot. In yet another alternate second mode, the differentialfeedback subchannel and the re-synch subchannel are not sent in parallelat all; the re-synch subchannel operates first over M slots, and thedifferential feedback subchannel operates over the next N−M slots of theN-slot frame. FIG. 2B and FIG. 2C are timelines illustrating thetransmission timing of the re-synch subchannel and the differentialfeedback subchannel operating in the second mode. The internal registerof the remote station may be updated in the first, second or M^(th)slot, depending on which operating mode is in use.

In another embodiment, the full C/I value can also be sent atunscheduled slots, whenever the remote station determines that the C/Iestimate kept at the base station is out of synchronization. Thisembodiment requires that the base station continuously monitors theCQICH to determine whether an unscheduled full C/I value symbol ispresent or not.

In yet another embodiment, the full C/I value is only sent when theremote station determines that the C/I estimate kept at the base stationis out of synchronization. In this embodiment, the full C/I value is notsent at regularly scheduled intervals.

Interpretation of Subchannel Information at a Base Station

A scheduling element in a base station can be configured to interpretchannel information received on the re-synch subchannel and thedifferential feedback subchannel, wherein the channel information fromeach subchannel is used to make transmission decisions that account forthe state of the channel. The scheduling element can comprise aprocessing element coupled to a memory element, and is communicativelycoupled to the receiving subsystem and the transmission subsystem of thebase station.

FIG. 3 is a block diagram of some of the functional components of a basestation with a scheduling element. A remote station 300 transmits on thereverse link to a base station 310. At a receiving subsystem 312, thereceived transmissions are de-spread, demodulated and decoded. Ascheduler 314 receives a decoded C/I value and orchestrates theappropriate transmission formats, power levels, and data rates oftransmissions from the transmission subsystem 316 on the forward link.

At the remote station 300, a receiving subsystem 302 receives theforward link transmission and determines the forward link channelcharacteristics. A transmission subsystem 306, in which the channelelements described by FIGS. 6A and 6B are located, transmits suchforward link channel characteristics to the base station 310.

In the embodiments described herein, the scheduling element 314 can beprogrammed to interpret the channel information received on the re-synchsubchannel together with the channel information received on thedifferential feedback subchannel, or to interpret the channelinformation received on the re-synch subchannel separately from thechannel information received on the differential feedback subchannel.The scheduling element can also be configured to perform a method toalternate which subchannel will be used to update channel information.

When the remote station transmits the channel information using thefirst mode, a serving base station will receive the full C/I value overone slot and incremental values over all slots of the frame. In oneembodiment, the scheduler can be programmed to reset internal registersthat store the current state of the channel, wherein the registers arereset with the full C/I value received over one slot of the re-synchsubchannel. The incremental values received over the different feedbacksubchannel are then added upon receipt to the full C/I value stored inthe register. In one aspect, the incremental value that is transmittedconcurrently over the slot with the full C/I value is intentionallydiscarded, since the full C/I value already accounts for thisincremental value.

When a remote station is operating in the second mode, a serving basestation will receive the full C/I value over multiple slots andincremental values over all slots of the frame. In one embodiment, theserving base station estimates the channel conditions at the time thatis scheduled for a packet transmission by accumulating the incrementalvalues received on the differential feedback subchannel from the secondslot to the M^(th) slot, where M is the number of slots over which thefull C/I value is spread out. This accumulated value is then added tothe full C/I value, which was received on the re-synch subchannel overthe M slots. In another embodiment, this “accumulate and add” method canbe performed concurrently with an independent action for “up-down” bits,which updates the C/I value stored in the register as directed by theincremental values. Hence, the register storing the current channelcondition information is updated each time an incremental value isreceived, and the register is then updated with the accumulated valueadded to the full C/I value.

FIGS. 4A, 4B, 4C and 4D are timelines describing the embodiments above.FIG. 4E is a table of C/I values stored in a register at a given pointin the timelines, using the embodiments described above. In the timelineof FIG. 4A, the remote station is transmitting the re-synch subchannelover a single slot of the CQICH frame and the differential subchannelover each slot of the CQICH frame. The base station is configured toupdate the register that stores the channel state such that parallelincremental values are discarded, i.e., the parallel incremental valuesare not used to update the register. Hence, at the interval t₂–t₃, thechannel state information stored in the register is 4 dB, which is thefull C/I value transmitted on the re-synch subchannel over intervalt₁–t₂. The contribution of the differential feedback channel at intervalt₁–t₂ is discarded.

In the timeline of FIG. 4B, the remote station is transmitting there-synch subchannel over multiple slots (4 slots in this example) andthe differential subchannel over each slot of the CQICH frame. Again,the base station is configured to update the register that stores thechannel state such that parallel incremental values are discarded.Hence, at the interval t₁–t₅, the channel state information stored inthe register is 11 dB, which is the value of the different feedbacksubchannel over interval t₀–t₁ added to the stored value in theregister. The register is not updated with the value carried by there-synch subchannel until t₅, which is the instance when the re-synchC/I value has been fully received.

In the timeline of FIG. 4C, the remote station is transmitting there-synch subchannel over a single slot and the differential subchannelover each slot of the CQICH frame. In this example, one of the benefitsof the embodiments described herein can be shown clearly. From intervalt₀–t₁, the last value in the register is 10 dB. From interval t₁–t₂, thevalue in the register is 11 dB. If the re-synch subchannel can bedecoded correctly, then the register values over the intervals t₂–t₃ andt₃–t₄ would be the same as for the timeline in FIG. 4A. However, if there-synch subchannel cannot be decoded correctly, then the registervalues over the intervals t₂–t₃ and t₃–t₄ would be 10 dB and 11 dB,respectively, rather than 4 dB and 5 dB. Even though, the full C/I valueis lost on the re-synch subchannel, the incremental values received onthe differential feedback subchannel can still be used to update theregister. Hence, the differential feedback subchannel can be usedindependently of the re-synch subchannel to update the channel stateinformation registers.

In the timeline of FIG. 4D, the remote station is transmitting there-synch subchannel over multiple slots (4 slots in this example) andthe differential subchannel over each slot of the CQICH frame. The basestation is configured to update the register that stores the channelstate, wherein the update accounts for the addition of parallelincremental values to the stored C/I re-synch value, as each incrementalvalue arrives at the base station.

In an alternative embodiment, the base station can be configured toupdate the register that stores the channel state, wherein the updateincludes the accumulation of the parallel incremental values that isthen added to the stored C/I re-synch value. In particular, theaccumulate and add is performed using all incremental values except forthe incremental value transmitted in the first shared slot with the fullC/I value. Each parallel incremental value is added to the stored C/Ire-synch value as each arrives, and the aggregate of the incrementalvalues, except the first, is added to the newly received C/I value att₅.

The embodiments described above serve the practical purpose of allowingthe base station to more closely model the event of a deep fade.Rayleigh fading, also known as multipath interference, occurs whenmultiple copies of the same signal arrive at the receiver in adestructive manner. Substantial multipath interference can occur toproduce flat fading of the entire frequency bandwidth. If the remotestation is travelling in a rapidly changing environment, deep fadescould occur at scheduled transmission times. When such a circumstanceoccurs, the base station requires channel information that allows it toreschedule transmissions quickly and accurately. In the second mode, thebase station receives a reduced rate C/I value over more than one slot,but the base station can still compensate for the fade before the C/Ivalue is fully received over the multiple slots. FIG. 5 is a deep fadingcurve superimposed over a timeline that can be used to illustrate thepurpose of this embodiment.

At time t₀, a deep fading condition commences. Due to incremental stepcommands, the base station slowly models the fade, as shown by thedouble-dashed line. At time t₁, the remote station transmits a measuredC/I ratio at a reduced rate over multiple slots of the re-synchsubchannel. The remote station concurrently transmits incremental “up”commands on each slot to the base station. The base station startsdemodulating and decoding the C/I value on the re-synch subchannel.Since the 1-bit “up” command is relatively simple to demodulate anddecode, the base station can immediately start modeling the fade usingthe received up commands. At time t₂, wherein the C/I value is fullyprocessed, the base station resets its estimate of the channelconditions.

As shown by FIG. 5, without the use of the differential feedbackchannel, the base station would have continued to pursue a model ofchannel conditions that is sub-optimal. Rather than a model with apositive slope between the points t₁ and t₂, the model would have had anegative slope between points t₁ and t₂. Moreover, using the “accumulateand add” method, the base station would be able to estimate a highervalue of the channel state than the one already provided by the remotestation. Hence, the base station would have had a model that would beless accurate then the model created by the current embodiments.

The use of two subchannels as described above allows the base station toreact to the changing environment in which the remote station isoperating while minimizing the reverse link load. The reverse link loadis reduced because the majority of the slots will be carrying fewerinformation bits than continuous transmissions of full C/I values. Forexample, in the case of the second mode, one full C/I value is beingconveyed over all N slots of the CQICH frame, rather than thetransmission of N full C/I values over N slots.

FIG. 6A is a block diagram of channel elements that can implement themodes described above in a cdma2000 1×EV-DV system. C/I ratio values 601are input into an encoder 602 at rate R=4/12 so that 12 binary symbolsare generated for each slot. The 12 binary symbols are spread with aWalsh code generated by a covering element 612. Covering element 612selects one of six allowed spreading Walsh sequences based on coversymbols 610 to indicate the index of the serving base station. Theoutput of the covering element 612 and the encoder 602 are combined byan adder 604 to form 96 binary symbols per slot. The output from theadder 604 is mapped in a mapping element 606 and then spread by a Walshspreading element 608 to generate the re-synch subchannel 600.Concurrently, incremental 1-bit values 621 are input into a repeater 622to form 96 binary symbols per slot. The repeated symbols are mapped in amapping element 624 and then spread by a Walsh spreading element 626 toform the differential feedback subchannel 620. The symbols sent on there-synch and the differential feedback subchannels are transmitted at arate of 1.2288 Mcps.

FIG. 6B is an alternate configuration wherein the concurrent incremental1-bit values 621 are input into a repeater 622 to form 12 binary symbolsper slot. The rationale for this alternate configuration is discussedbelow in conjunction with the new transition indicator subchannel 630.

Base Station Index Indicator

The Walsh spreading introduced by covering element 612 of FIG. 6A servesthe purpose of indicating the best base station detected by the remotestation, i.e. the base station with the highest forward link C/I value,for the purposes of packet-based transmissions. It should be noted thatthe process of choosing a best base station for packet-basedtransmissions on a data traffic channel is different from the process ofchoosing the best base station for voice transmissions on a fundamentalchannel. For a voice transmission, a remote station that transitionsfrom the range of a first base station to a second base station willexchange voice traffic with both base stations at the same time in aprocess called soft handoff. Each base station operating within thenetwork is assigned a 20-bit identification value, and is ranked ingroups referred to as the active set, the candidate set, the neighborset, and the remaining set. Due to the variable nature of wirelessmedium, the ranking of base stations is a dynamic process.

The embodiments described herein are directed to data traffic channelsthat exchange packets directed to individual base stations, due to thenature of addressed packet data. In order to select the best basestation to serve the remote station, the remote station monitors forwardlink signals from all base stations in a designated “active set.” Asused herein, the “active set” for a packet-based transmission differsfrom the “active set” for a voice transmission.

Each member of the active set is assigned a different 3-bit index, whichis conveyed to the remote station from the serving base station throughsignaling messages. The Walsh code to be used by covering element 612 isselected based on the index corresponding to the best base station inthe active set. In FIG. 6A and FIG. 6B, the Walsh spreading is appliedonly to the re-synch subchannel and not to the differential feedbacksubchannel. This embodiment has the advantage of conserving Walsh-space,since only differential subchannel symbols are sent for a majority ofthe slots. Thus, the Walsh functions are used infrequently and areresources that can be directed to other purposes. In one aspect of thisembodiment, the extra Walsh function is applied to a transitionindicator subchannel, which is described below.

In another embodiment, the Walsh spreading is applied to both there-synch subchannel and the differential feedback subchannel, thus thebase station index indicator can be extracted from either.

In another embodiment, one of the Walsh functions is reserved forspreading the differential feedback subchannel symbols, while theremaining Walsh functions are used for spreading the re-synch subchannelsymbols to indicate the best base station index. This embodiment has thedisadvantage of reducing the number of available active set base stationindices by one. However, this embodiment provides for straightforward,concurrent use of the re-synch subchannel and the differential feedbacksubchannel, since they are spread with mutually orthogonal codes.

As a further advantage, when the new best base station is a differentsector of the current serving base station, then the switching ofsectors can be immediate. The remote station can start sending re-synchsubchannel and differential feedback subchannel symbols corresponding tothe new best base station immediately.

When the new best base station is a sector of a different base station,a transitional period for allowing a new forward link to be set up isdesirable. In one embodiment, channel elements are configured togenerate a transition indicator subchannel. The transition indicatorsubchannel is set up so that a remote station can generate re-synchsubchannel symbols and differential feedback subchannel symbols thatcorrespond to the current base station's C/I value. This allows theremote station to utilize the existing forward link from the currentbase station. The transition indicator subchannel is shown in FIG. 6Aand FIG. 6B. Concurrent to the re-synch subchannel and the differentialfeedback subchannel, mismatch flag bits 631 are input into a repeater632 to form 48 binary symbols per slot in FIG. 6A and 12 binary symbolsper slot in the alternate configuration of FIG. 6B. The repeated symbolsare mapped in a mapping element 634 and then spread by a Walsh spreadingelement 636 to form the transition indicator subchannel 630. FIG. 6Aillustrates a transition indicator subchannel with Walsh function W₂₈³², while FIG. 6B illustrates a transition indicator subchannel withWalsh function W₂₈ ¹²⁸. The symbols sent on this subchannel aretransmitted at a rate of 1.2288 Mcps.

The transition indicator subchannel indicates the start of atransitional period to the current base station. The transitional periodis indicated by setting a bit in the transition indicator subchannel.The transition indicator subchannel may be transmitted in acode-multiplexed or time-multiplexed fashion. Code-multiplexing of thetransition indicator subchannel with other existing subchannels may beperformed through the use of a reserved Walsh spreading function.

In one embodiment, the transitional period is indicated by using a Walshspreading function that is the inverse of the Walsh spreading functionassigned to a base station in the non-transitional case. As used herein,the inverse means using ‘0’ in place of ‘1’ and using ‘1’ in place of‘0’ in the Walsh sequence. This embodiment requires that the union ofthe set of all code words generated by encoder 602 of FIG. 6A or FIG. 6Band the set of inverses of all such code words forms a codebook that hassatisfactory minimum distance properties. To achieve this, anappropriate encoder 602 must be used. One such possible encoder isobtained by puncturing the first four bits of a standard 16×16 Walshcode.

In one embodiment, all re-synch subchannel symbols are transmitted at areduced rate throughout the switching period to aid reliable detectionof the switch from the current base station to a new base station. Toimprove time diversity in fading channels, the reduced rate repetitionsmay be performed in non-consecutive slots. This aspect of the embodimentreduces C/I tracking performance by introducing additional delays in thefull C/I update, but increases the reliability of detecting the basestation index indicator, which is of higher importance.

Interpretation of Subchannel Information at Quantization Limits

As stated above, the C/I value is transmitted as 4 bits of information;hence, there are only 16 possible values for the C/I value. The dynamicrange of this quantization scheme is a system-defined parameter that canbe altered without affecting the scope of the embodiments, i.e., more orless bits can be allocated for the dynamic range of the C/I values. Asone illustrative example, one quantization scheme can be defined ashaving a minimum bit sequence value “0000” set equal to −15.5 dB and amaximum bit sequence value “1111” set equal to 5.5 dB. A question arisesas to the appropriate course of action at these two extremes.

Using the above embodiments, if the channel conditions are extremelyfavorable at 8 dB over a long period of time, then the only value thatthe re-synch subchannel can transmit is 5.5 dB. The remote station canattempt to compensate for this lack by transmitting incremental “up”bits to the base station. However, the base station is not likely totake different actions for a channel condition of 5.5 dB versus 8 dB.Moreover, the decoding errors accumulated during the “above the limit”period will add to the tracking error even after the C/I value dropsbelow the maximum quantization level.

In one aspect of the embodiments above, the base station candeliberately ignore the values received on the differential feedbacksubchannel when a threshold C/I value is reached and a predeterminedpattern of transmissions on the differential feedback subchannel isdetected. In one example, a remote station determines that the conditionof the forward link is better than the maximum quantization value and sotransmits the maximum quantization value over the re-synch subchannel.In addition, the remote station deliberately transmits up bits to theserving base station throughout the duration that this favorable channelcondition exists. The transmission of up bits only is contrary to thepractice of transmitting up and down bits to track the slope of thefading curve. Referring back to FIG. 5, if the fading curve is above thethreshold amount at intervals t₁–t₃, then up bits would have been sentin interval t₁–t₂, and down bits would have been sent in interval t₂–t₃.However, using the embodiment described herein, only up bits would havebeen sent in the intervals t₁–t₂ and t₂–t₃.

The base station decodes the full C/I value on the re-synch subchanneland determines that the full C/I value is equal to the threshold value,which corresponds to the maximum value of the dynamic range. If the basestation then receives any up bits, the base station is programmed torefrain from updating the registers that store the current channelconditions until a full C/I value is received that is not the thresholdvalue. However, if the base station receives down bits, then the basestation updates the registers accordingly.

In an additional embodiment, the remote station determines that thecondition of the forward link is worse than the minimum quantizationvalue and so transmits the minimum quantization value over the re-synchsubchannel. In addition, the remote station deliberately transmits downbits to the serving base station throughout the duration that thisunfavorable channel condition exists. The base station decodes the fullC/I value on the re-synch subchannel and determines that the full C/Ivalue is equal to the threshold value, which corresponds to the minimumvalue of the dynamic range. If the base station then receives any downbits, the base station is programmed to refrain from updating theregisters that store the current channel conditions until another fullC/I value is received that does not match the threshold value. However,if the base station receives up bits, then the base station updates theregisters accordingly.

FIG. 7 illustrates the benefit of these embodiments. A fading curve isshown against a threshold value×dB. If the fade dips below thethreshold, then the remote station transmits the representation of thethreshold value×dB on the re-synch subchannel and down bits on thedifferential feedback subchannel. If the down bits where taken intoaccount, then a situation arises wherein up bits could be transmittedbefore the transmission of the full C/I value on the re-synchsubchannel. The estimate of the fade would follow line 700 until there-synch message is received at point t_(re-synch). However if the downbits were not taken into account, then the transmission of up bits wouldcommence at point t_(up). The estimate of the fade would follow line 710until the re-synch message is received at point t_(re-synch). As one mayobserve, line 710 is a better approximation of the fading condition thanline 700. Hence, implementation of this embodiment improves the abilityof the base station to track the channel conditions.

The use of a threshold for updating the channel state informationregisters has an additional benefit: the effects of bit errors on thedifferential feedback subchannel are mitigated because the base stationcan be configured to recognize the pattern of constant up bits orconstant down bits on the differential feedback subchannel. In otherwords, if the threshold value is transmitted, and the incremental valuesare constant for the duration that the threshold value is exceeded, thenthe base station will know that an occasional, isolated bit that isdifferent from the expected, constant stream of bits is an error.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An apparatus for transmitting channel quality values over a feedbackchannel to a base station, comprising: a re-synch subchannel generationsystem for generating a full channel quality value; and a differentialfeedback subchannel generation system for generating a plurality ofincremental values, wherein the plurality of incremental values aremultiplexed with the full channel quality values; and a transmitter fortransmitting the full channel quality value and the plurality orincremental values over multiple slots in a frame, wherein the pluralityof incremental values are discarded by the base station.
 2. Theapparatus of claim 1, wherein the plurality of incremental values arecode-multiplexed with the full channel quality value.
 3. The apparatusof claim 1, wherein the plurality of incremental values aretime-multiplexed with the full channel quality value.
 4. The apparatusof claim 1, further comprising a transition indicator subchannelgeneration system for generating a flag that indicates the start of atransitional period.
 5. The apparatus of claim 4, wherein a Walshspreading element is used in the re-synch subchannel generation systemand not used in the differential feedback subchannel.
 6. The apparatusof claim 4, wherein a common Walsh function is used in the differentialfeedback subchannel generation system and the transition indicatorsubchannel generation system.
 7. The apparatus of claim 6, wherein thecommon Walsh function is used to indicate a base station index.
 8. Amethod for transmitting channel information from a remote station to abase station, comprising: generating a full channel quality value; andgenerating a plurality of incremental channel quality values, whereinthe incremental channel quality values are multiplexed with the fullchannel quality value; and transmitting the full channel quality valueand the plurality of incremental values over multiple slots in a frame,wherein the plurality of incremental values are discarded birdie basestation.
 9. The method of claim 8, wherein the incremental channelquality values are generated over each slot in the frame.
 10. The methodof claim 9, further comprising: generating a transition indicator,wherein the transition indicator is multiplexed with the incrementalchannel quality values and the full channel quality value and is used toindicate a transition period for the base station.
 11. Apparatus fortransmitting channel information from a remote station to a basestation, comprising: means for generating a full channel quality value;means for generating a plurality of incremental channel quality values,wherein the incremental channel quality values are multiplexed with thefull channel quality value; and means for transmitting the full channelquality value and the plurality of incremental values over multipleslots in a frame, wherein the plurality of incremental values arediscarded by the base station.
 12. The apparatus of claim 11, furthercomprising: means for generating a transition indicator, wherein thetransition indicator is multiplexed with the incremental channel qualityvalues and the full channel quality value and is used to indicate atransition period for the base station.
 13. A method for transmittingchannel information from a remote station to a base station, comprising:generating a full channel quality value and determining said fullchannel quality value exceeds a fixed maximum value represented by afinite number of data bits; generating an incremental channel qualityvalue and fixing said incremental value as an up-increment value,wherein said fixing is in response to said determining; multiplexing theincremental channel quality value with the full channel quality value toform channel information for transmission from said remote station tosaid base station.
 14. A method for transmitting channel informationfrom a remote station to a base station, comprising: generating a fullchannel quality value and determining said full channel quality value isbelow a fixed minimum value represented by a finite number of data bits;generating an incremental channel quality value and fixing saidincremental value as a down-increment value, wherein said fixing is inresponse to said determining; multiplexing the incremental channelquality value with the full channel quality value to form channelinformation for transmission from said remote station to said basestation.
 15. An apparatus for transmitting channel information from aremote station to a base station, comprising: means for generating afull channel quality value and determining said full channel qualityvalue exceeds a fixed maximum value represented by a finite number ofdata bits; means for generating an incremental channel quality value andfixing said incremental value as an up-increment value, wherein saidfixing is in response to said determining; means for multiplexing theincremental channel quality value with the full channel quality value toform channel information for transmission from said remote station tosaid base station.
 16. An apparatus for transmitting channel informationfrom a remote station to a base station, comprising: means forgenerating a full channel quality value and determining said fullchannel quality value is below a fixed minimum value represented by afinite number of data bits; means for generating an incremental channelquality value and fixing said incremental value as a down-incrementvalue, wherein said fixing is in response to said determining; means formultiplexing the incremental channel quality value with the full channelquality value to form channel information for transmission from saidremote station to said base station.